This invention relates to a silent discharge type pulse laser device in which the electrical voltage applied to the silent discharge section for exciting the pulse laser has a period longer than the period for silent discharge and has a peak value for a fraction of said period which is sufficient to cause the discharge but insufficient to excite the pulse laser.
FIG. 1 shows a pulse laser device typical of the various devices of the prior art. In this figure, the reference numeral 1 denotes a metallic electrode, and the reference numeral denotes 2 a dielectric electrode (i.e.--an electrode having a dielectric cover) designed to produce silent discharge between it and the metallic electrode 1, and having a metallic part 21 and a dielectric cover 22. The reference numeral 3 denotes a silent discharge section, the reference numeral 4 denotes a transformer, the reference numeral 5 denotes a high frequency source, the reference 6 denotes a total reflection mirror, and the reference numeral 7 denotes a partial reflection mirror forming a laser resonator with the total reflection mirror 6. The reference numerals 8 and 9 denote arrow marks respectively indicating the laser gas and laser output. The laser gas employed is a Co.sub.2 --N.sub.2 --He gas mixture with a mol fraction of 5-60-35 and a pressure of about 200 Torr.
The silent discharge pulse lase device operates as follows. When an electrical voltage having a high frequency and amplitude as shown in FIG. 2(a) is applied by the source 5 and transformer 4 across the electrodes 1 and 2, a silent discharge takes place in the space therebetween. The voltage applied is a sinusoidal voltage having a period T0 of 10 .mu.s corresponding to the silent discharge period. This voltage is applied with a peak value V1 equal to 8 kV and for a time interval T1 equal to 0.5 ms, and no voltage is applied for the following period T2 of 0.5 ms. Then, laser oscillation is caused by the laser gas excited by the silent discharge section. Since the duration of the excited laser molecules is about 100 .mu.s, the laser oscillation output has a pulse-like waveform as shown in FIG. 2(b). This laser oscillation output is a series of pulses having a period equal to T1 and T2 and a pulse width Tp shorter than the period T1 and approximately equal to 0.3 ms. The pulse buildup is more gradual than the buildup of the electrical voltage applied for the period T1 because the laser gas is excited in the space of the silent discharge section 3 and it takes some time until the density of excited molecules is raised sufficiently to cause the laser oscillation. With continued operation of the pulse laser, the mean value of the laser oscillation output is decreased gradually as may be noticed from FIG. 2(c) illustrating a marked decrease of the mean output as a result of an operation for several hours.
The reason for such a decrease of the laser oscillation output is now considered by referring to FIG. 3 which shows schematically the discharge space of the silent discharge section 3 from the optical axis of laser light. As is evident from this figure, discharge streaks 32 in the form of silk threads of high brightness are scattered among the uniform glow discharge streaks 31 of deep blue to purple color. These streaks 32 represent a locally pinched discharge with locally elevated values of power density, electron density and gas molecule temperature. It has been shown by the testing and study of changes of gas composition by gas chromatography that, in the case of the occurrence of discharge streaks 32 of higher brightness, the CO.sub.2 in the laser gas is decreased abruptly with a passage of time whereas CO and O.sub.2 are increased. It may be assumed that the chemical change EQU CO.sub.2 .fwdarw.CO+1/2O.sub.2
takes place in the discharge space in an accelerated manner due to the presence of discharge streaks 32 of higher brightness to cause the decrease in laser oscillation output.
It has also been shown experimentally that, in the case of continued oscillation in the silent discharge section 3, apparition of the streak-like discharge as shown at 32 may be suppressed almost completely and thus the decrease of the laser oscillation output may not be noticed even when the laser device has operated for a prolonged time resulting in the stable laser output continuing for a prolonged time.
However, when a pulse laser oscillation is caused in the silent discharge section 3, there is evidently a certain relationship between the status of discharge occurring in the silent discharge space and the decrease of laser oscillation output. The present inventors were the first to discover and clarify the presence of such a relationship.
Thus, in summary, the prior-art silent discharge type pulse laser has a drawback that, as stated above, the laser output is decreased gradually with a passage of time.